Fix random probability check in ziplistRandomPairsUnique (#8467)
Fix random probability check in ziplistRandomPairsUnique (#8467)

When (remaining == (total_size - index)), element will definitely be random to.
But when rand() == RAND_MAX, the element will miss, this will trigger assert
in serverAssert(ziplistRandomPairsUnique(zsetobj->ptr, count, keys, vals) == count).
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